High voltage isolator for liquid feedline



y 3, 1 6 H. J. KING ET AL 3,443,570

HIGH VOLTAGE ISOLATOR FOR LIQUID FEEDLINE Filed Jan. 5, 1967 Jrrmay' United States Patent 3,443,570 HIGH VOLTAGE ISOLATOR FOR LIQUID FEEDLINE Harry J. King, Canoga Park, Jerome H. Molitor, Woodland Hills, and James W. Ward, Santa Monica, Calif.,

assignors to Hughes Aircraft Company, Culver City,

Calif., a corporation of Delaware Filed Jan. 3, 1967, Ser. No. 606,611 Int. Cl. F17d 3/00, 3/04 US. Cl. 137-1 6 Claims ABSTRACT OF THE DISCLOSURE High voltage isolation between the ends of the liquid feedline is provided by injecting a bubble of insulative material into the feedline. The method preferably includes sensing the existence of a previously injected bubble at a downstream point and using the signal to inject another bubble. It also includes removing the insulating bubble from the feedline. The apparatus includes a bulbble injector, a downstream bubble sensor connected to drive the bubble injector and a bubble remover downstream from the bubble sensor.

The invention described herein was made in the performance of work under a NASA con-tract and is subject to the provisions of Sec. 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (7 2 Stat. 435; 42 USC 2457).

Background This invention relates to fluid feed systems and more particularly to a method and apparatus for providing electrical isolation between the ends of a fluid feedline adapted to feed conductive fluid.

In an ion engine system, it is important to provide electrical isolation between individual thrustors so that faults or failures in one engine are not reflected throughout the complete array of thrustors. Further, it is desirable to electrically isolate the thrustors from their feed system and tankage in order to allow a single propellant reservoir or manifold tied to spacecraft ground to supply all the thrustors. When using mercury electron bombardment thrustors incorporating liquid mercury cathodes, the feedlines are filled with liquid mercury which is a good conductor.

Summary According to the preferred embodiment of the present invention, electrical isolation is accomplished by injecting an insulating gas bubble into the conductive liquid in the feedline at a point adjacent the inlet of the feedline, removing the gas bubble from the feedline at a point adjacent the outlet thereof, sensing the presence of a bubble as it approaches the outlet, and control-lably injecting another bubble into the line in response to a signal from the sensor indicating the presence of such a bubble, there assuring at least one bubble in the line at all times. The system of the present invention serves to interrupt or open the conductive path which previously existed in the feedline. Any voltage drops along the feedline will appear entirely across the gas bubble. The insulating properties of the gas bubbles are determined from the Paschen curve for the particular gas employed. For all pressures above torr, an air bubble of .5 cm. length can hold off several kilovolts without breakdown.

It is a primary object of the present invention to provide a high voltage isolater for a conductive fluid feedline. It is another object of the present invention to provide a method and apparatus for providing electrical isolation between the ends of the liquid feedline when the line is filled with the liquid which is a good conductor.

3,443,570 Patented May 13, 1969 Brief description of the drawings The figure. is a cross sectional, partly schematic view of a preferred embodiment of the present invention.

Descriptionl The invention will be described below by reference to the preferred embodiment thereof in which the feedline is used to carry liquid mercury to an ion thruster. The figure shows a liquid mercury feedline 2 for supplying liquid mercury propellant from a common manifold or propellant reservoir (not shown) to a liquid mercury cathode incorporated in a liquid mercury electron bombardment ion thrustor (not shown). The wall of the feedline 2 should be constructed of an insulating material such as glass, ceramic or Teflon, which will not be wet 'by the conductive liquid. This will aid in maintaining the configuration of the figure where the two sections of the conducting fluid are completely separated by the gas bubble. According to the present invention gas bubbles are injected into the mercury 4 which is flowing through the feedline 2 as follows. A gas reservoir 6 supplies the insulating gas through a gas flow line 8 provided with suitable one-way valves 10 and 12 which assure gas flow in the proper direction. The gas flow line 8 is connected to the feedline 2 at an opening 14 therein adjacent the inlet of the line 2. Gas bubbles 15 are injected into the liquid 4 from the gas reservoir 6 by an electromagnetically driven bellows 16 which forms a contractable chamber. At a point of the feedline 2 downstream from the point where the gas bubbles 15 are injected and adjacent the outlet of the line 2, a portion of the feedline 2 is formed with a porous-walled section 18. The material forming the porous wall can be, for example, porous tungsten. The porous section 18 is surrounded by an evacuated housing 20, provide with a pump 22 for maintaining the vacuum. If the feedline 2 exists in a vacuum, the housing 20 and pump 22 can be dispensed with. The material of which the porous section 18 is composed must be such as to allow the passage therethrough of the gas which is used to form the gas bubbles while preventing the passage therethrough of the liquid 4.

While the injection of the gas bubbles into the liquid 4 can be controlled in any one of a number of ways, such as manually or at predetermined intervals by the use of a simple timer-actuated pump or syringe, a preferred automatic method is shown in the figure. If, as in an ion thrustor system, the cathode is at a high positive potential (several thousand volts) and the mercury reservoir is at ground potential, then a coaxial capacitor 24 (which can be plated onto the outside of the feedline 2) can be used to sense the change in potential when a bubble passes through that section of the feedline 2 surrounded by the capacitor 24. The capacitor 24 is connected to ground through two resistors R and R connected in series. The capacitor 24 is connected to a pulser 26 through the junction between R, and R The pulser 26 thus receives a signal each time a bubble 15 passes through the capacitor 24. The pulser 26 is connected to solenoid 30 which is activated by the pulser 26 upon receiving a triggering signal from the capacitor 24. Air (other gases can be used) was used as the insulating gas forming the bubbles 15 and it was found that bubbles a few millimeters long could withstand the full voltage of the available power supply (11 kv.). No gas bubbles were visible downstream from the porous section of the feedline 2. The pulser can take a variety of configurations depending on the exact application. One embodiment is to use a thyratron or SCR, which is triggered by the signal from the capacitor 24, in series with a capacitor and the coil of the solenoid. The capacitor is charged by an auxiliary circuit. When triggered, the thyratron or other switch allows a pulse of current to pass through'the' solenoid thus driving bubble of gas into the feedline 2.

Other methods for controlling the injection of gas bubbles into the feedline can be used. In the timer regulating method mentioned above, it would be desirable to regularly inject bubbles into the feedline at predetermined time intervals in those applications in which the rate of flow of the fluid through the feedline is known. Although the embodiment shown in the figure provides for essentially one bubble in the feedline at one time, clearly more than one bubble can be used in the feedline at any one time. Another method of controlling the injection of bubbles rather than the one shown in the figure is to control the injection of bubbles in response to the measured rate of flow of the fluid. The material forming the bubble must be such with respect to the fluid (liquid of gas) flowing through the feedline that it will form a bubble rather than merely dissolve or become assimilated into the liquid. The material forming the bubble must also be such with respect to the material forming the wall of the feedline that the bubble will freely flow through the line and not adhere to the wall.

In the preferred embodiment of the invention, it is necessary to remove the bubble from the feedline to prevent it from affecting the operation of the cathode. In other applications of the present invention in which the presence of the fluid material of which the bubble is composed would not aflect the operation of the device, there is no necessity to remove the bubble. An insulating liquid can be used to form a bubble for use in the present invention rather than a gas as described above with respect to the preferred embodiment. In such a case if the liquid forming the insulating bubble does not affect the operation of the feed device, it can be left in the feedline. If it does affect the operation of the device, it can be removed by a simple valve for draining the portion of the fluid containing the bubble. The opeartion of the valve can be controlled by a simple bubble-sensing means upstream of the valve Another method of removing the insulating material forming the bubble from the feedline is to use an insulating material which is either more or less dense than the fluid being fed through the line and to feed the fluid into a centrifugal separator.

The present invention is not limited in use to the preferred embodiment described above involving high power ion thrustor systems but is useful in any application involving the flow of a conductive fluid through a conduit in which application it is desired to electrically isolate the two ends of the feedline.

We claim: 1. A method for providing electrical isolation between the ends of a fluid feedline composed of insulating material and adapted to carry conductive fluid, the improvement comprising:

injecting, into said feedline, sufi'icient insulating fluid material to form an insulating bubble of said insulating fluid material which extends .across the entire width of said feedline so as to interrupt the electrical continuity of said conductive fluid;

sensing the existence of a previously injected bubble in said feedline at a predetermined point downstream from the injection point in said feedline;

injecting another bubble into said feedline upstream from the sensing point when a bubble is sensed at said predetermined downstream point; and

removing said insulating fluid material bubble from said feedline at a point downstream of the point at which said bubble is injected into said feedline and downstream of the sensing point.

2. The method according to claim 1 in which said insulating fluid material is a gas and in which said removing step comprises passing said gas through a porous wall section of said feedline, said porous section allowing the passage therethrough of said gas but preventing the passage therethrough of said conductive material.

3. In a liquid mercury ion thrustor system the method of electrically isolating individual thrustors connected to a liquid mercury propellant feedline, the improvement comprising:

injecting, into the liquid mercury feedline, a quantity of insulating gas suflicient to form a gas bubble extending across the entire width of the feedline so as to interrupt the electrical continuity of the mercury in the feedline, said bubble being adapted to flow downstream with the liquid mercury, removing the bubble from the liquid mercury in the feedline at a point downstream from the point at which the bubble is injected into the feed line by passing the bubble through a porous walled section of the feedline, said porous walled section allowing the passage therethrough of the gas but preventing the passage therethrough of liquid mercury, sensing, at a point adjacent to but upstream from the point at which the bubble is removed from the feedline, the existence of the bubble in the feedline, and

injecting upstream from the sensing point another bubble into the feedline each time a bubble is so sensed.

4. Apparatus for electricaly isolating the ends of a fluid feed line, the improvement comprising:

an insulating fluid feedline adapted to carry a conductive fluid;

injector means for injecting an insulating fluid into said line adjacent the inlet of the line, which insulating fluid is adapted to form a bubble in the conductive fluid so as to interrupt the electrical continuity of the conductive fluid;

sensing means positioned downstream from said injector means for detecting a bubble in the feedline adjacent said detecting means, said sensing means being connected to said injector means so that when said detecting means detects a bubble in said feedline said injector means injects another bubble in said feedline; and

means for removing the insulating fluid forming the bubble from said line.

5. The apparatus according to claim 4 in which said removing means comprises a porous walled section of the feedline encased in an evacuated container, the porous wall allowing the passage therethrough of the insulating fluid but preventing the passage therethrough of the conductive fluid.

6. Apparatus for electrically isolating individual thrustors connected to liquid mercury propellant feedlines in a liquid mercury ion thrustor system, the improvement comprising:

injector means for injecting a quantity of insulating gas sufiicient to form a gas bubble extending across the entire width of the feedline so as to interrupt the electrical continuity of the mercury in the feedline, said bubble being adapted to flow downstream with the liquid mercury, said injector means comprising a contractible chamber containing insulating gas and means to contract said chamber to expell insulating gas from said chamber and inject it into the feedline,

means for removing the insulating gas bubble from the liquid mercury in the feedline at a point downstream from the point at which the bubble is injected into the feedline, said removing means comprising a porous-walled section of the feedline surrounded by an evacuated container, said porous-walled section allowing the passage therethrough of the gas but preventing the passage therethrough of liquid mercury; and

5 6 sensing means for sensing, at a point adjacent to but 2,694,404 11/1954 Alexander 1371 upstream from the point at which the bubble is re- 2 9 5 114 12 1960 Harden 137 1 moved from the feedline the existence of a bubble 3,033,911 5/1962 D u d dy 137 197 X in the feedhne, said sensing means being connected 3 1 6 Sk X to said means to contract said chamber so that when 09714 11 19 3 eggs a bubble is sensed by said sensing means, said chamher is contracted to inject another bubble in said ALAN COHAN, Examine"- feedline to continue to interrupt electrical continuity.

References Cited UNITED STATES PATENTS 2,627,933 2/1953 Teter 55-158 US. 01. X.R. 

